This invention relates to a catalytic process for the hydroconversion of heavy hydrocarbon streams containing asphaltenic material, metals, and sulfur compounds. More particularly, this invention relates to hydroconversion using multiple-stage catalytic treatment with catalysts having improved effectiveness and activity maintenance in the desulfurization of metal-containing hydrocarbon streams.
As refiners increase the proportion of heavier, poorer quality crude oil in the feedstock to be processed, the need grows for processes to treat the fractions containing increasingly higher levels of metals, asphaltenes, and sulfur.
It is widely known that various organometallic compounds and asphaltenes are present in petroleum crude oils and other heavy petroleum hydrocarbon streams, such as petroleum hydrocarbon residua, hydrocarbon streams derived from tar sands, and hydrocarbon streams derived from coals. The most common metals found in such hydrocarbon streams are nickel, vanadium, and iron. Such metals are very harmful to various petroleum refining operations, such as hydrocracking, hydrodesulfurization, and catalytic cracking. The metals and asphaltenes cause interstitial plugging of the catalyst bed and reduced catalyst life. The various metal deposits on a catalyst tend to poison or deactivate the catalyst. Moreover, the asphaltenes tend to reduce the susceptibility of the hydrocarbons to desulfurization. If a catalyst, such as a desulfurization catalyst or a fluidized cracking catalyst, is exposed to a hydrocarbon fraction that contains metals and asphaltenes, the catalyst will become deactivated rapidly and will be subject to premature replacement.
Although processes for the hydroconversion of heavy hydrocarbon streams, including but not limited to heavy crudes, reduced crudes, and petroleum hydrocarbon residua, are known, the use of fixed-bed catalytic processes to convert such feedstocks without appreciable asphaltene precipitation and reactor plugging and with effective removal of metals and other contaminants, such as sulfur compounds and nitrogen compounds, are not common because the catalysts employed have not generally been capable of maintaining activity and performance.
Thus, the subject hydroconversion processes are most effectively carried out in an ebullated bed system. In an ebullated bed, preheated hydrogen and resid enter the bottom of a reactor wherein the upward flow of resid plus an internal recycle suspend the catalyst particles in the liquid phase. Recent developments involved the use of a powdered catalyst which can be suspended without the need for a liquid recycle. In this system, part of the catalyst is continuously removed in a series of cyclones and fresh catalyst is added to maintain activity. Roughly about 1 wt. % of the catalyst inventory is replaced each day in an ebullated bed system. Thus, the overall system activity is the weighted average activity of catalyst varying from fresh to very old i.e., deactivated.
Hopkins et al., in U.S. Pat. No. 4,119,531, disclose a process for hydrodemetallation of hydrocarbon streams containing asphaltenes and a substantial amount of metals, which comprises contacting the hydrocarbon stream with a catalyst consisting essentially of a small amount of a single hydrogenation metal from Group VIB or Group VIII, deposed on a large pore alumina; suitable examples of the hydrogenation metal are nickel or molybdenum. The catalyst is characterized by a surface area of at least 120 m.sup.2 /gm; a pore volume of at least 0.7 cc/gm and an average pore diameter of at least 125 .ANG. units.
Hensley et al., in U.S. Pat. No. 4,297,242, disclose a multiple-stage catalytic process for hydrodemetallation and hydrodesulfurization of heavy hydrocarbon streams containing asphaltenes and a substantial amount of metals. The first stage of this process comprises contacting the feedstock in a first reaction zone with hydrogen and a demetallation catalyst comprising hydrogenation metal selected from Group VIB and/or Group VIII deposed on a large-pore, high surface area inorganic oxide support; the second stage of the process comprises contacting the effluent from the first reaction zone with a catalyst consisting essentially of hydrogenation metal selected from Group VIB deposed on a smaller pore, catalytically active support comprising alumina, said second stage catalyst having a surface area within the range of about 150 m.sup.2 /gm to about 300 m.sup.2 /gm, an average pore diameter within the range of about 90 .ANG. to about 160 .ANG., and a pore volume within the range of about 0.4 cc/gm to about 0.9 cc/gm. Hensley et al. disclose that as little as 2.2 wt. % cobalt oxide caused more rapid deactivation of their second-stage catalyst for sulfur removal.
In U.S. Pat. No. 4,212,729 to Hensley et al., another two-stage catalytic process for hydrodemetallation and hydrodesulfurization of heavy hydrocarbon streams containing asphaltenes and metals is disclosed. In this process, the first-stage demetallation catalyst comprises a metal selected from Group VIB and from Group VIII deposed on a large-pore, high surface area inorganic oxide support. The second stage catalyst contains a hydrogenation metal selected from Group VIB deposed on a smaller pore catalytically active support having the majority of its pore volume in pore diameters within the range of about 80 .ANG. to about 130 .ANG..
Other examples of multiple-stage catalytic processes for hydrotreatment of heavy hydrocarbon streams containing metals are disclosed in U.S. Pat. Nos. 3,180,820 (Gleim et al., 1965); 3,730,879 (Christman, 1973); 3,977,961 (Hamner, 1976); 3,985,684 (Arey, et al., 1977); 4,016,067 (Fischer, 1977); 4,054,508 (Milstein, 1977); 4,051,021 (Hamner, 1977); and 4,073,718 (Hamner, 1978).
The catalysts disclosed in these references contain hydrogenating components comprising one or more metals from Group VIB and/or Group VIII on high surface area support such as alumina, and such combinations of metals as cobalt and molybdenum, nickel and molybdenum, nickel and tungsten, and cobalt, nickel, and molybdenum have been found useful. Generally, cobalt and molybdenum have been preferred metals in the catalysts disclosed for hydrotreatment of heavy hydrocarbon streams, both in first-stage catalytic treatment to primarily remove the bulk of the metal contaminants, and in second-stage catalytic treatment primarily for desulfurization.
A difficulty which arises in resid hydroconversion units employing the above catalyst systems is the formation of insoluble carbonaceous substances also known as Shell hot filtration solids. These substances cause operability problems in the hydroconversion units. Certain resids tend to produce greater amounts of solids thereby limiting the level of upgrading by the amount of these solids the hydroprocessing unit can tolerate.
Further, the higher the conversion level for given feedstocks, the greater the amount of solids formed. In high concentrations, these solids accumulate in lines and separators, causing fouling, and in some cases interruption or loss of process flow.
Accordingly, it is a general object of this invention to provide a process affording a higher conversion level for heavy hydrocarbon feedstocks that tend to form greater amounts of insoluble substances, especially that fraction of the feedstock that boils over 1,000.degree. F.
It is another object of the present invention to provide a process that can tolerate larger amounts of insoluble carbonaceous substance producing feedstocks in the feed stream to the process.
It is yet another object of the present invention to provide a process that employs a less expensive catalyst system than conventional processes designed to handle heavy hydrocarbon feed streams.
These objectives can be attained by the process of the present invention involving a multi-stage process for hydrodemetallation, hydrodesulfurization, and conversion of a hydrocarbon feedstock containing asphaltenes and a substantial amount of metals. The process of the present invention differs from the above-cited demetallation-de sulfurization processes in that the desulfurization catalyst employed in the process of the invention contains macropores such that the pore volume of pores having a diameter larger than 1,200 .ANG. ranges from about 0.1 to about 0.25 cc/gm. This is in marked contradistinction to the desulfurization catalysts disclosed in U.S. Pat. Nos. 4,212,729 and 4,297,242 wherein less than 15% of the total pore volume resides in pores having diameters above 130 .ANG.. Further, the process of the present invention also requires the presence of a Group VIII metal in addition a Group VIB metal in the desulfurization catalyst in contradistinction to U.S. Pat. Nos. 4,212,729 and 4,297,242 which limit the active hydrogenation metal in the desulfurization catalyst to one selected from Group VIB metals.
Additionally, while the desulfurization catalyst used in the process of the invention has the subject macropore volume suitable for demetallation of the feedstock it is a relatively expensive catalyst since it contains relatively large amounts of Group VIB and Group VIII metals. The process of the present invention supplants a portion of the desulfurization catalyst with a relatively less expensive demetallation catalyst containing the requisite macropore volume for demetallation, but a lower amount of hydrogenation metals. The overall system is then relatively less expensive than using only the desulfurization catalyst to effect both demetallation and desulfurization.
This feature of the invention is illustrated in the following manner. If one assumes that the hydrodemetallation process adheres to a first-order kinetics model and that there is an overall metals removal by the catalyst of 95%, one obtains a characteristic distribution of metals in the catalyst bed. This distribution is such that the first third of a catalyst bed would have 65.7% of the total amount of metals removed from an influent stream, the middle third would have 24.7%, and the last third would have only 9.5% of the metals. Thus, if one uses a catalyst that has the capacity to hold 60% of its weight in metals, the first third would hold 60 wt. % metals, the second third would hold 22.6 wt. % metals while the last third would hold only 8.6% metals, far below the catalyst's 60 wt. % capacity. The entire system would have to be shut down prior to the last two-thirds of the bed reaching their capacity for demetallation. However, if only the first third of the catalyst bed is supplanted with a high capacity demetallation catalyst having a capacity to hold 100% of its weight in metals, the second third will then hold 37.6% of its weight in metals and the last third 14.4%. Thus, by placing a high capacity catalyst in the first bed only, the overall average capacity increases from 30% of all of the catalyst's weight in metals holding capacity to 50.6%. Or, the overall life of the system is increased by 68% prior to shut down.
Also, while providing a less expensive overall catalyst system, the process of the present invention provides a catalyst system that can handle feeds that produce large amounts of insoluble carbonaceous substances or alternatively effect a high conversion level for feedstocks that produce relatively less insoluble carbonaceous substances because both catalysts in the invention system posses the requisite macropore volume instrumental in reducing the formation of insoluble carbonaceous substances.